I'll be honest up front. I don't have, nor have I had, a ferret. I have interacted with them and I know they're smart, full of energy when they're awake, and like to tunnel.
I can't tell you for sure that the ferret won't kill the ground squirrels anyway. If it doesn't, then enough harassment might cause them to leave, but more than likely, they'll just go back down into their holes once you're gone.
Anyway, I think the easiest method is going to be to use clicker training. Teach your ferret a few minor cues, like coming to you when you call. This will be important if you are letting him loose outside.
Practice in the house with corrugated pipe. Buy a few short sections and some T-joints from home depot or whatever home and garden store you have nearby. You'll need something scented like the ground squirrel. Whether it's ground squirrel hide or something else you can find doesn't matter.
Start by putting a few treats in the tunnel. Send the ferret in to find them. When he comes out of the tunnel and comes back to you, click and reward him with a high value treat. Higher value than the treats in the tunnel. So you could put kibbles in the tunnel and give him chicken when he comes back to you. What you're trying to train is that you'll send him into the tunnel and he needs to come back to you for his reward. Going into the tunnel is a fun activity and coming back to you is the real reward.
Start with a straight piece of tunnel, but once he gets the idea, put on a T-joint and add some more pipe. At first, you should be putting treats all over the tunnels, but eventually you'll want to reduce the amount of treats in the tunnel so he has to look for them more. It'll also be important that you eventually send him in and there are no treats in there. You'll have to teach him it's okay to no find anything. He should come back to you and you'll have a treat every time.
You should also keep track of how much food he'll take before it doesn't seem to matter to him. He'll get full after a while and play will be more important than food rewards. He might not come back to you easily after this happens. You'll want to take him out before he eats, and quite sending him into holes before he's full.
Once he's going after the treats, start replacing it with the ground squirrel lure. Again, start off with just the straight tunnel. Put the lure inside one end with a string on it. Send him in the other ends. Maybe put a couple of treats before the lure. Let him sniff it good, then start pulling it out in short jerky motions. Don't pull it too fast, too soon. When he follows the lure out, take it away and call him back to you for a treat. The object isn't to catch the lure, but to chase it out of the tunnel. You can vary this theme. You can pull the lure back past him, so that it goes under his belly or over his back. You ad sections and pull it around corners.
After a while, he'll understand chasing out the lure leads to a good treat. You can always toss a treat in the entrance of an outside hole to get him started.
To move the training outside, dig a trench an bury the corrugated pipe, with the ends sticking up. You don't have to bury it deep and you can even start by just laying it on the ground at first. At some point you do want to teach him to go down into the ground, though. To advance his training, I'd bury a length of tube, then I'd put a box in the ground. You should be able to lift the lid off and where the pipe comes up to it on either side should be screen and you should be able to left them up. This will allow you to set a live trap and catch a ground squirrel. Put the squirrel in the box and have both the screen doors down. Send in the ferret. When you think he's at the door, open the escape side for the squirrel and then open the ferrets side. He'll chase it out the other side and then you can treat him with chicken.
If you're worried about him running off outside, you can do a couple of things. One thing is you can put a short fence around the 'training' area. He could escape if left alone, but you can easily catch him when he comes out of the tunnel. The other thing is to leash him and spend time outside letting him become used to it. If the only time he gets out is when he's hunting, then he'll be easily distracted. However, if going outside is just an ordinary thing, then he'll concentrate on work.
This is the best idea I can come up with. Good luck.